GPS Satellites broadcast navigation data modulated on the L1 and L2 carrier frequencies. The data contains coarse ephemeris data (referred to as almanac data) for all satellites in the constellation, precise ephemeris data for this particular satellite, and timing data and model correction parameters needed by a GPS receiver to make a correct ranging measurement. The satellites also broadcast two forms of ranging codes: the Coarse/Acquisition code (C/A), which is freely available to the public, and the restricted Precise code (P-code), which is usually reserved for military applications.
GPS receivers receive the clock data to know the precise time of the signal transmission not only in the timescale of the satellite but also in the timescale of the satellite system. Using that time, they calculate position, velocity, and time solutions (PVT) correctly. For this reason, the satellites are equipped with extremely accurate atomic clocks. Most receivers use an internal crystal oscillator-based clock that is continually updated using the signals from the satellites.
The receiver identifies each satellite's signal by its distinct spreading code pattern and then measures the time delay in generating that spreading code pattern for each satellite. To do this, the receiver produces an identical spreading sequence using the same seed number and shift register setups as the satellite. By lining up the two sequences, the receiver can determine a pseudo-range, the difference of the time of reception in the timescale of the receiver and of the time of transmission in the system or satellite timescale, or in other words the measurement of delay and calculation of the distance to the satellite.
Calculating a position with the P(Y) signal is conceptually similar in that a receiver must first decrypt the signal, then use information in the navigation data to know where in the week-long pseudorandom noise (PRN) sequence the satellite is currently broadcasting. Once this is known, tracking and measurement are the same. The encryption of P code into Y code is essentially a security mechanism; it is reasonable to assume that if a signal can be successfully decrypted, it is a real signal being sent by a GPS satellite and not a “spoofed” signal. A spoofed GPS signal is an overwritten (spoofed) signal. A spoof is defined as a malicious signal that overpowers the authentic signal and misleads the receiver to use a forged signal for further processing. In contrast, civil receivers are highly vulnerable to spoofing, since correctly formatted C/A signals can be generated using readily available signal generators.
A GPS receiver, however, can never measure exact range to each satellite because the measurement process is corrupted by noise which introduces errors into the calculation. This noise includes errors in the ionospheric corrections and system dynamics not considered during the measurement process (e.g., user clock drift). A Kalman filter characterizes the noise sources in order to minimize their effect on the desired receiver outputs.
When the GPS receiver is aided or integrated with other navigation sensors (e.g., inertial navigation sensors (“INS”), clock, or altimeter), then the Kalman filter can be extended to include the measurements added by these sensors. For more accurate position measurements, a user receiver receives deviation information from a reference receiver and to provide differential correction to the user receiver. A system using a user receiver with one or more reference receivers is referred to as Differential GPS (DGPS). Examples of differential reference systems are RTCM, StarFire, WAAS, LAAS, EGNOS, and MSAT.
The idea of differential positioning is to correct range bias errors at the mobile receiver location with the observed range bias errors at a known position. The reference station computes corrections for each satellite signal. DGPS implementations require software in the reference receiver that can track all “visible” satellites and form pseudo-range corrections. These corrections are transmitted to the user receiver, which applies these corrections to the pseudo-range measurement for each satellite used in the navigation solution. In this case, the reference receiver has limited effect at useful ranges because both receivers would have to be using the same set of satellites to resolve their navigation solutions.
Current DGPS systems may be “spoofed” with erroneous data from the reference receiver that will confuse the DGPS receiver. When the DGPS receiver receives erroneous data, that data can cause the DGPS receiver to report position or velocity vectors that contain hazardously misleading errors with respect to the true values of the vectors. In short, the DGPS may produce a value for either or both vectors that is less accurate than those reported by the non-differential GPS.
European Patent document EP 2 146 217 A1 describes a method and apparatus for calculating corrections to a navigation solution based on differential GPS data which includes receiving GPS ephemeris from at least three GPS satellites. A position, velocity, and time (PVT) solution is resolved from the GPS ephemeris. The PVT solution includes a Circular Error Probable (CEP). Differential GPS data for calculating the corrections to the PVT solution is received. A corrected PVT solution is then based upon the differential GPS data. The corrected PVT solution is compared to a region defined by the CEP. Where the corrected PVT solution is not within the region, the corrected PVT solution is rejected in favor of the PVT solution for determining an accurate navigational solution.
A disadvantage of EP2 146 217 A1 is that by using the CEP 50% of all measurements are outside of the CEP, therefore the availability of the system is very low. With the HUL of Brenner availability could be increased to 99.9%, but the HUL is not mentioned in the claims. Furthermore the HUL is very bad in absorbing biases. Another disadvantage of the proposed solution in EP 2 146 217 A1 is the non-conservative estimation of the probability of the corrected PVT solution being further than the alert limit away from the real position.
Exemplary embodiments of the present invention involve a method with a higher availability than the methods described in the prior art and which provides a conservative estimation of the risk that the corrected position solution is further than the alert limit away from the real position.
One embodiment of the invention describes a method of calculating corrections to a navigation solution based on accurate data which comprises the steps of receiving GNSS ephemeris, clock models and other navigation information from at least three GNSS satellites; performing pseudo-ranging to the GNSS satellites; resolving a PVT solution from the GNSS ephemeris, clock models and other navigation information, and the pseudo range measurement, wherein the PVT solution includes a statistical measure; receiving differential GNSS data for calculating the corrections to the PVT solution calculating a corrected PVT solution based upon the differential GNSS data; comparing the corrected PVT solution to an region defined by the statistical measure; and rejecting the corrected PVT solution where the corrected PVT solution is not within the region.
One technical effect is that this method provides a higher availability than the methods described in the prior art. A further advantage is the method provides a conservative estimation of the risk that the corrected position solution is further than the alert limit away from the real position.
According to another embodiment of the invention the statistical measure is based on protection levels.
According to a further embodiment of the invention the statistical measure is based on integrity risk.
According to another embodiment of the invention the rejecting of the corrected PVT solution includes activating an alert.
According to a further embodiment of the invention the activating of an alert includes modifying an icon in a display.
According to another embodiment of the invention the activating of an alert includes generation of a display including words and figures indicative of rejection of the differential GPS data.
According to a further embodiment of the invention the activation of an alert includes an aural alert.
Another embodiment of the invention describes an apparatus and a system for calculating corrections to a navigation solution based on accurate data, comprising: means for receiving GNSS ephemeris, clock models and other navigation information from at least three GNSS satellites; means for performing pseudo-ranging to the GNSS satellites; means for resolving a PVT solution from the GNSS ephemeris, clock models and other navigation information and the pseudo range measurements, wherein the PVT solution includes a statistical measure; means for receiving differential GNSS data for calculating the corrections to the PVT solution calculating a corrected PVT solution based upon the differential GNSS data; means for comparing the corrected PVT solution to an region defined by the statistical measure; and means for rejecting the corrected PVT solution where the corrected PVT solution is not within the region.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.